White, biaxially-oriented polyester film with cycloolefin copolymer (COC), which is matt on at least one side, method for production and use thereof

ABSTRACT

The invention relates to a white, biaxially-oriented and coextruded polyester film which is matt on at least one side with at least one base layer B, which contains a cycloolefin copolymer (COC) in amounts of 4 to 60 wt. %, based on the weight of the base layer, in addition to polyester starting material. The glass transition temperature T g  of the COC lies in the range from 70 to 270° C. Said film has at least one matt outer layer (A), made from polyester and is particularly suitable for use in fast running machines such as winding, metallising, printing or laminating machines.

The present invention relates to a white, biaxially oriented andcoextruded polyester film with at least one matt side and embracing atleast one base layer and at least one matt outer layer, where at leastthe base layer comprises a polyester and a cycloolefin copolymer (COC).The invention further relates to the use of the polyester film and to aprocess for its production.

BACKGROUND OF THE INVENTION

White, biaxially oriented polyester films are known from the prior art.These known prior-art films are either easy to produce, or have goodoptical properties, or have acceptable processing performance.

DE-A 2 353 347 describes a process for producing a milky polyester filmhaving one or more layers, which comprises preparing a mixture fromparticles of a linear polyester with from 3 to 27% by weight of ahomopolymer or copolymer of ethylene or propylene, extruding the mixtureas a film, quenching the film and biaxially orienting the film viaorientation in directions running perpendicular to one another, andheat-setting the film. A disadvantage of this process is that regrindwhich arises during production of the film (essentially a mixture ofpolyester and ethylene or propylene copolymer) cannot be reused withoutyellowing the film. This makes the process uneconomic, but theyellow-tinged film produced with regrind could not gain acceptance inthe market.

EP-A 0 300 060 describes a single-layer polyester film which comprises,besides polyethylene terephthalate, from 3 to 40% by weight of acrystalline propylene polymer and from 0.001 to 3% by weight of asurface-active substance. The effect of the surface-active substance isto increase the number of vacuoles in the film and at the same time toreduce their size to the desired extent. This gives the film greateropacity and lower density. A residual disadvantage of the film is thatregrind which arises during production of the film (essentially amixture of polyester and propylene homopolymer) cannot be reused withoutyellowing the film. This makes the process uneconomic, but theyellow-tinged film produced with regrind could not gain acceptance inthe market.

EP-A 0 360 201 describes a polyester film having at least two layers andcomprising a base layer with fine vacuoles, with a density of from 0.4to 1.3 kg/dm³, and having at least one outer layer whose density isabove 1.3 kg/dm³. The vacuoles are achieved by adding from 4 to 30% byweight of a crystalline propylene polymer, followed by biaxialstretching of the film. The additional outer layer improves the ease ofproduction of the film (no streaking on the film surface), and thesurface tension is increased and the roughness of the laminated surfacecan be reduced. A residual disadvantage is that regrind arising duringproduction of the film (essentially a mixture of polyester and propylenehomopolymer) cannot be reused without yellowing the film. This makes theprocess uneconomic, but the yellow-tinged film produced with regrindcould not gain acceptance in the market.

EP-A 0 795 399 describes a polyester film having at least two layers andcomprising a base layer with fine vacuoles, the density of which is from0.4 to 1.3 kg/dm³, and having at least one outer layer, the density ofwhich is greater than 1.3 kg/dm³. The vacuoles are achieved by addingfrom 5 to 45% by weight of a thermoplastic polymer to the polyester inthe base, followed by biaxial stretching of the film. The thermoplasticpolymers used are, inter alia, polypropylene, polyethylene,polymethylpentene, polystyrene or polycarbonate, and the preferredthermoplastic polymer is polypropylene. As a result of adding the outerlayer, ease of production of the film is improved (no streaking on thefilm surface), the surface tension is increased and the roughness of thelaminated surface can be matched to prevailing requirements. Furthermodification of the film in the base layer and/or in the outer layers,using white pigments (generally TiO₂) and/or using optical brighteners,permits the properties of the film to be matched to the prevailingrequirements of the application. A residual disadvantage is that regrindwhich arises during production of the film (essentially a mixture ofpolyester and the added polymer) cannot be reused without undefined andhighly undesirable changes in the color of the film. This makes theprocess uneconomic, but the discolored film produced with regrind couldnot gain acceptance in the market.

DE-A 195 40 277 describes a polyester film having one or more layers andcomprising a base layer with fine vacuoles, with a density of from 0.6to 1.3 kg/dm³, and having planar birefringence of from −0.02 to 0.04.The vacuoles are achieved by adding from 3 to 40% by weight of athermoplastic resin to the polyester in the base, followed by biaxialstretching of the film. The thermoplastic resins used are, inter alia,polypropylene, polyethylene, polymethylpentene, cyclic olefin polymers,polyacrylic resins, polystyrene or polycarbonate, preferred polymersbeing polypropylene and polystyrene. By maintaining the stated limitsfor the birefringence of the film, the film claimed has in particularsuperior tear strength and superior isotropy properties. However, aresidual disadvantage is that regrind arising during production of thefilm cannot be reused without undefined discoloration of the filmarising, and this in turn is highly undesirable. This makes the processuneconomic, but the colored film produced with regrind could not gainacceptance in the market.

The object of the present invention was to provide a white, biaxiallyoriented polyester film which has at least one matt side and is easierto produce, i.e. has low production costs. The intention was inparticular to ensure that regrind arising directly during the productionprocess can be reused for the production process in amounts of from 10to 70% by weight, based on the total weight of the film, without anysignificant resultant adverse effect on the physical or opticalproperties of the film produced using regrind. In particular, theaddition of regrind is intended to cause no significant yellowing of thefilm.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, the object is achieved by way of a white,biaxially oriented polyester film with at least one matt side andcomprising at least one base layer B and at least one matt outer layerA, both made from polyester, the characteristic features of the filmbeing that at least the base layer B also comprises an amount in therange from 2 to 60% by weight, based on the weight of the base layer B,of cycloolefin copolymer (COC), the glass transition temperature T_(g)of the cycloolefin copolymer (COC) being in the range from 70 to 270° C.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, a white, biaxially orientedpolyester film is a film of this type whose whiteness is above 70%,preferably above 75%, and particularly preferably above 80%. The opacityof the film of the invention is moreover above 55%, preferably above60%, and particularly preferably above 65%.

To achieve the desired whiteness of the film of the invention, theamount of cycloolefin copolymer (COC) in the base layer B is intended tobe greater than 2% by weight, otherwise the whiteness is below 70%. If,on the other hand, the amount of COC is greater than 60% by weight itbecomes impossible to produce the film cost-effectively, since reliablestretching of the film becomes impossible.

It is also necessary for the glass transition temperature T_(g) of theCOC used to be above 70° C. Otherwise, if the glass transitiontemperature T_(g) of the COC used is below 70° C. the raw materialmixture has poor processability because it becomes difficult to extrude.The desired whiteness is lost and the regrind used gives a film with atendency toward increased yellowing. If, on the other hand, the glasstransition temperature T_(g) of the selected COC is above 270° C. itbecomes impossible to achieve adequate homogenization of the rawmaterial mixture in the extruder. This then results in a film withundesirably inhomogeneous properties.

In the preferred embodiment of the film of the invention, the glasstransition temperature T_(g) of the COCs used is in a range from 90 to250° C., and in the particularly preferred embodiment it is in a rangefrom 110 to 220° C.

Surprisingly, it has been found that the addition of a COC in the mannerdescribed above can produce a white, opaque film.

The whiteness and the opacity of the film can be adjusted precisely andadapted to the prevailing requirements as a function of the amount andthe nature of the COC added. By using this measure it is possible todispense substantially with any use of other familiar whitening andopacifying additives.

None of these features described was foreseeable. All the more so, sincealthough COCs appear to be substantially incompatible with polyethyleneterephthalate they are known to be oriented using stretching conditionsand stretching temperatures which are similar to those for polyethyleneterephthalate. In these circumstances the skilled worker would not haveexpected to be able to produce a white, opaque film under theseproduction conditions.

In the preferred and the particularly preferred embodiments, the film ofthe invention has high and, respectively, particularly high whitenessand high and, respectively, particularly high opacity, while the changeof film color resulting from regrind addition remains extremely small.

The film of the invention is a multilayer film. Multilayer embodimentshave at least two layers and always embrace the COC-containing baselayer B and at least one matt outer layer A. In one preferredembodiment, the COC-containing layer forms the base layer B of the filmwith at least one outer layer or with outer layers on both sides, and,where appropriate, there may be (an) intermediate layer(s) present onone or both sides. In another preferred embodiment, the COC-containinglayer also forms an intermediate layer of the multilayer film. Otherembodiments with COC-containing intermediate layers have a five-layerstructure with, alongside the COC-containing base layer B,COC-containing intermediate layers on both sides. In another embodiment,in addition to the base layer, the COC-containing layer may form (an)outer layer(s) on one side or both sides, on the base layer orintermediate layer. For the purposes of the present invention, the baselayer is the layer which makes up more than from 30 to 99.5%, preferablyfrom 70 to 95%, of the entire thickness of the film. The outer layer isthe layer which forms the outermost layer of the film.

Each embodiment of the invention is a non-transparent, opaque film. Forthe purposes of the present invention, non-transparent films are filmswhose light transmittance to ASTM D1003-77 is below 95%, preferablybelow 75%.

The COC-containing layer (the base layer B) of the film of the inventioncomprises a polyester, preferably a homopolyester, a COC, and, whereappropriate, other additives, each in effective amounts. This layergenerally comprises at least 20% by weight, preferably from 40 to 96% byweight, in particular from 70 to 96% by weight, of polyester, based onthe weight of the layer.

The base layer B of the film comprises a thermoplastic polyester.Polyesters suitable for this purpose are those made from ethylene glycoland terephthalic acid (=polyethylene terephthalate, PET), from ethyleneglycol and naphthalene-2,6-dicarboxylic acid (=polyethylene2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclohexane andterephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate,PCDT), or else from ethylene glycol, naphthalene-2,6-dicarboxylic acidand biphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesterscomposed of at least 90 mol %, preferably at least 95 mol %, of ethyleneglycol units and terephthalic acid units or ethylene glycol units andnaphthalene-2,6-dicarboxylic acid units. The remaining monomer units arederived from other aliphatic, cycloaliphatic, or aromatic diols and,respectively, dicarboxylic acids.

Examples of other suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloaliphaticdiols, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic diols arethose of the formula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—,—C(CF₃)₂—, —O—, —S— or —SO₂—. Bisphenols of the formulaHO—C₆H₄—X—C₆H₄—OH are also highly suitable.

Other preferred aromatic dicarboxylic acids are benzenedicarboxylicacids, naphthalenedicarboxylic acids (such as naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid), andstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids mention should be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the (C₃-Cl₉)-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

An example of a method for preparing the polyester is thetransesterification process. The starting materials here aredicarboxylic esters and diols, and these are reacted using the usualtransesterification catalysts, such as salts of zinc, of calcium, oflithium, of magnesium or of manganese. The intermediates are thenpolycondensed in the presence of typical polycondensation catalysts,such as antimony trioxide or titanium salts. They may equally well beprepared by the direct esterification process in the presence ofpolycondensation catalysts, starting directly from the dicarboxylicacids and the diols.

According to the invention, the COC-containing layer comprises an amountof not less than 2.0% by weight, preferably from 4 to 50% by weight andparticularly preferably from 6 to 40% by weight, of a cycloolefincopolymer (COC), based on the weight of the layer or, in the case ofsingle-layer embodiments, based on the weight of the film. It issignificant for the present invention that the COC is not compatiblewith the polyethylene terephthalate and does not form a homogeneousmixture with the same in the melt.

Cycloolefin polymers are homopolymers or copolymers which containpolymerized cycloolefin units and, if desired, acyclic olefins ascomonomer. Cycloolefin polymers suitable for the present inventioncontain from 0.1 to 100% by weight, preferably from 10 to 99% by weight,particularly preferably from 50 to 95% by weight, of polymerizedcycloolefin units, in each case based on the total weight of thecycloolefin polymer. Particular preference is given to polymers whichhave been built up using the monomers comprising the cyclic olefins ofthe formulae I, II, III, IV, V or VI:

R¹, R², R³, R⁴, R⁵, R⁶, R⁷ and R⁸ in these formulae are identical ordifferent and are a hydrogen atom or a C₁-C₃₀-hydrocarbon radical, ortwo or more of the radicals R¹ to R⁸ have been bonded cyclically, andthe same radicals in the different formulae may have the same or adifferent meaning. Examples of C₁-C₃₀-hydrocarbon radicals are linear orbranched C₁-C₈-alkyl radicals, C₆-C₁₈-aryl radicals, C₇-C₂₀-alkylenearylradicals and cyclic C₃-C₂₀-alkyl radicals and acyclic C₂-C₂₀-alkenylradicals.

If desired, the COCs may contain from 0 to 45% by weight, based on thetotal weight of the cycloolefin polymer, of polymerized units of atleast one monocyclic olefin of the formula VII:

n here is a number from 2 to 10.

If desired, the COCs may contain from 0 to 99% by weight, based on thetotal weight of the COC, of polymerized units of an acyclic olefin ofthe formula VIII:

R⁹, R¹⁰, R¹¹ and R¹² here are identical or different and are a hydrogenatom or a C₁-C₁₀-hydrocarbon radical, e.g. a C₁-C₈-alkyl radical or aC₆-C₁₄-aryl radical.

Other polymers suitable in principle are cycloolefin polymers which areobtained by ring-opening polymerization of at least one of the monomersof the formulae I to VI, followed by hydrogenation.

Cycloolefin homopolymers have a structure composed of one monomer of theformulae I to VI. These cycloolefin polymers are less suitable for thepurposes of the present invention. Polymers suitable for the purposes ofthe present invention are cycloolefin copolymers (COC) which comprise atleast one cycloolefin of the formulae I to VI and acyclic olefins of theformula VIII as comonomer. Acyclic olefins preferred here are thosewhich have from 2 to 20 carbon atoms, in particular unbranched acyclicolefins having from 2 to 10 carbon atoms, for example ethylene,propylene and/or butylene. The proportion of polymerized units ofacyclic olefins of the formula VIII is up to 99% by weight, preferablyfrom 5 to 80% by weight, particularly preferably from 10 to 60% byweight, based on the total weight of the respective COC.

Among the COCs described above, those which are particularly preferredcontain polymerized units of polycyclic olefins having a fundamentalnorbornene structure, particularly preferably norbornene ortetracyclododecene. Particular preference is also given to COCs whichcontain polymerized units of acyclic olefins, in particular ethylene.Particular preference is in turn given to norbornene-ethylene copolymersand tetracyclododecene-ethylene copolymers which in each case containfrom 5 to 80% by weight, preferably from 10 to 60% by weight, ofethylene (based on the weight of the copolymer).

The cycloolefin polymers generically described above generally haveglass transition temperatures T_(g) in the range from −20 to 400° C.However, COCs which can be used for the invention have a glasstransition temperature T_(g) above 70° C., preferably above 90° C. andin particular above 110° C. The viscosity number (decalin, 135° C., DIN53 728) is advantageously from 0.1 to 200 ml/g, preferably from 50 to150 ml/g.

The COCs are prepared by heterogeneous or homogeneous catalysis withorganometallic compounds, as described in a wide variety of documents.Suitable catalyst systems based on mixed catalysts made from titaniumcompounds and, respectively, vanadium compounds in conjunction withaluminum organyl compounds are described in DD 109 224, DD 237 070 andEP-A-0 156 464. EP-A-0 283 164, EP-A-0 407 870, EP-A-0 485 893 andEP-A-0 503 422 describe the preparation of COCs with catalysts based onsoluble metallocene complexes. The preparation processes for COCsdescribed in the abovementioned specifications are expresslyincorporated herein by way of reference.

The COCs are incorporated into the film either in the form of puregranules or in the form of granulated concentrate (masterbatch), bypremixing the polyester granules or polyester powder with the COC or,respectively, with the COC masterbatch, followed by feeding to anextruder. In the extruder, the mixing of the components continues andthey are heated to the processing temperature. It is advantageous herefor the novel process if the extrusion temperature is above the glasstransition temperature T_(g) of the COC, generally above the glasstransition temperature T_(g) of the COC by at least 5 K, preferably byfrom 10 to 180 K, in particular by from 15 to 150 K.

The advantageous embodiment of the matt outer layer A comprisesadditives in the form of inert inorganic antiblocking agents and, whereappropriate, a blend or a mixture of two components (I) and (II).

Typical antiblocking agents (in this context also termed pigments) areinorganic and/or organic particles, such as calcium carbonate, amorphoussilica, talc, magnesium carbonate, barium carbonate, calcium sulfate,barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, lithium fluoride, the calcium, barium, zincor manganese salts of the dicarboxylic acids used, carbon black,titanium dioxide, kaolin or crosslinked polymer particles, e.g.polystyrene particles or acrylate particles.

The additives selected may also comprise mixtures of two or moredifferent antiblocking agents or mixtures of antiblocking agents of thesame composition but different particle sizes. The particles may beadded to the polymers of the individual layers of the film in therespective advantageous amounts, e.g. as a glycolic dispersion duringthe polycondensation or via masterbatches during extrusion. Pigmentconcentrations which have proven particularly suitable are from 0 to 25%by weight (based on the weight of the respective layer). EP-A-0 602 964,for example, describes the antiblocking agents in detail.

Component (I) of the mixture or of the blend is an ethyleneterephthalate homopolymer or ethylene terephthalate copolymer, or amixture made from ethylene terephthalate homo- or copolymers.

Component (II) of the mixture or of the blend is an ethyleneterephthalate copolymer which is composed of the condensation product ofthe following monomers or of their derivatives capable of formingpolyesters:

-   -   A) from 65 to 95 mol % of isophthalic acid;    -   B) from 0 to 30 mol % of at least one aliphatic dicarboxylic        acid having the formula HOOC(CH₂)nCOOH, where n is in the range        from 1 to 11;    -   C) from 5 to 15 mol % of at least one sulfomonomer containing an        alkali metal sulfonate group on the aromatic moiety of a        dicarboxylic acid;    -   D) a copolymerizable aliphatic or cycloaliphatic glycol having        from 2 to 11 carbon atoms, in the stoichiometric amount        necessary to form 100 mol % of condensate;        each of the percentages given being based on the total amount of        the monomers forming component (II). For a detailed description        of component (II) reference is made to the content of EP-A 0 144        878, which is expressly incorporated herein by way of reference.

For the purposes of the present invention, mixtures are mechanicalmixtures prepared from the individual components. For this, theindividual constituents are generally combined in the form ofsmall-dimensioned compressed moldings, e.g. lenticular or bead-shapedpellets, and mixed with one another mechanically, using a suitableagitator. Another way of producing the mixture is to feed components (I)and (II) in pellet form separately to the extruder for the outer layerof the invention, and to carry out mixing in the extruder and/or in thedownstream systems for transporting the melt.

For the purposes of the present invention, a blend is an alloy-likecomposite of the individual components (I) and (II) which can no longerbe separated into the initial constituents. A blend has properties likethose of a homogeneous material and can therefore be characterized byappropriate parameters.

The ratio (ratio by weight) of the two components (I) and (II) of theouter layer mixture or of the blend can be varied within wide limits anddepends on the intended use of the multilayer film. The ratio ofcomponents (I) and (II) is preferably in the range from (I):(II)=10:90to (I):(II) 95:5, preferably from (I):(II)=20:80 to (I):(II) 95:5, andin particular from (I):(II)=30:70 to (I):(II)=95:5.

In one preferred embodiment, the matt outer layer A is characterized bythe following set of parameters:

-   -   The roughness of the matt outer layer A, expressed in terms of        R_(a), is in the range from 200 to 1000 nm, preferably from 220        to 950 nm and particularly preferably from 250 to 900 nm. Values        smaller than 200 nm have adverse effects on the mattness of the        surface, while values greater than 1000 nm impair the optical        properties of the film.    -   The value measured for gas flow should be in the range from 0 to        50 s, preferably from 1 to 45 s. At values above 50 s the        mattness of the film is adversely affected.

The base layer B and the other layers may also comprise conventionaladditives, such as stabilizers, antiblocking agents, and other fillers.The additives are usefully added to the polymer or to the polymermixture before melting begins. Examples of stabilizers used arephosphorus compounds, such as phosphoric acid or phosphoric esters.

To improve the whiteness of the film, the base layer or the otheradditional layers may comprise further pigmentation. It has provenparticularly advantageous here for the additional additives used tocomprise barium sulfate with grain size of from 0.3 to 0.8 μm,preferably from 0.4 to 0.7 μm, or titanium dioxide with grain size offrom 0.05 to 0.3 μm, in each case measured by the sedigraph method. Thisgives the film a brilliant white appearance. The amount of bariumsulfate or titanium dioxide is in the range from 1 to 25% by weight,preferably from 1 to 20% by weight, and very preferably from 1 to 15% byweight.

The total thickness of the film can vary within wide limits and dependson the intended application. The preferred embodiments of the film ofthe invention have total thicknesses from 4 to 400 μm, preferably from 8to 300 μm, in particular from 10 to 300 μm. The thickness(es) of anyintermediate layer(s) present is/are generally from 0.5 to 15 μm,independently of each other, and preference is given to intermediatelayer thicknesses of from 1 to 10 μm, in particular from 1 to 8 μm. Thevalues given are each based on one intermediate layer. The thickness(es)of the outer layer(s) is/are selected independently of the other layersand is/are preferably in the range from 0.1 to 10 μm, in particular from0.2 to 5 μm, with preference from 0.3 to 4 μm, and outer layers appliedto both sides may have identical or different thickness and composition.The thickness of the base layer B is correspondingly given by thedifference between the total thickness of the film and the thickness ofthe outer layer(s) and intermediate layer(s) applied, and can thereforevary similarly to the total thickness within wide limits.

The invention also provides a process for producing the polyester filmof the invention by extrusion or coextrusion methods known per se.

The procedure for this process is that the melts corresponding toindividual layers of the film are coextruded through a flat-film die,the resultant film is drawn off on one or more rollers forsolidification, the film is then biaxially stretched (oriented), and thebiaxially stretched film is heat-set and, where appropriate, corona- orflame-treated on the surface intended for treatment.

The biaxial stretching is usually carried out sequentially. For this, itis preferable to begin by stretching longitudinally (i.e. in the machinedirection=MD) and then to stretch transversely (i.e. perpendicularly tothe machine direction=TD). This orients the molecular chains. Thelongitudinal stretching preferably takes place with the aid of tworollers rotating at different peripheral speeds corresponding to thedesired stretching ratio. For the transverse stretching, use isgenerally made of an appropriate tenter frame.

The temperature at which the stretching is carried out may vary within arelatively wide range, and depends on the desired properties of thefilm. The longitudinal stretching is generally carried out at from 80 to130° C., and the transverse stretching at from 90 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1.

The stretching may also takes place in a simultaneous stretching frame(simultaneous stretching), and the number of stretching steps and thesequence (longitudinal/transverse) is not of decisive importance herefor the property profile of the film. However, advantageous stretchingtemperatures here are ≦125° C., particularly ≦115° C. The stretchingratios correspond to those in the conventional sequential process.

In the heat-setting which follows, the film is held at a temperature offrom 150 to 250° C. for from about 0.1 to 10 s. The film is then cooledand is wound up in the usual manner. However, before winding-up, thefilm may also be chemically treated, or else corona- or flame-treated toestablish other desired properties. The intensity of treatment isadjusted so that the surface tension of the film is generally above 45mN/m.

The film may also be coated to achieve other properties. Typicalcoatings are those with adhesive action, antistatic action,slip-improving action, or release action. These additional layers may,of course, be applied to the film by in-line coating, using aqueousdispersions, after longitudinal stretching and prior to transversestretching.

The particular advantage of the film of the invention is its highwhiteness and its high opacity. The whiteness of the film is above 70%,preferably above 75%, and particularly preferably above 80%. The opacityof the film of the invention is above 55%, preferably above 60%, andparticularly preferably above 65%. The gloss of the outer layer A isbelow 80, preferably below 70 and particularly preferably below 60.

Another particularly surprising advantage of the invention is thatduring production of the film it is possible for directly associatedregrind to be reused for the production process in amounts of from 10 to70% by weight, based on the total weight of the film, without anyresultant significant adverse effect on the physical properties of thefilm produced using the regrind. In particular, the regrind(substantially composed of polyester and COC) does not cause undefinedchange in the film color, whereas this is always the case with prior-artfilms.

A further advantage of the invention is that the production costs of thefilm of the invention are comparable with those for conventionalprior-art opaque films. The other properties of the film of theinvention relevant to its processing and use remain substantiallyunchanged, or indeed have been improved.

The film has excellent suitability for packaging for foods or otherconsumable items which are sensitive to light and/or to air. It also hasexcellent suitability for industrial use, e.g. in the production ofhot-stamping foils, or as a label film. The film is naturally alsoparticularly suitable for image-recording papers, printed sheets, ormagnetic recording cards, to mention just a few possible applications.

The film has outstandingly good processing and winding performance inparticular on high-speed machinery (winders, metallizers, printingmachines or laminating machines). One measure of processing performanceis the coefficient of friction of the film, and this is below 0.6. Agood thickness profile, excellent layflat, and low coefficient offriction affect winding performance, and the roughness of the film has adecisive effect on winding performance. It has been found that thewinding of the film of the invention is particularly good when theaverage roughness is in a range from 50 to 1000 nm while the otherproperties are retained. Methods of varying the roughness includevarying the COC concentration, the mattness of outer layer A, and theprocess parameters during the production process, within the rangegiven.

The table below (Table 1) gives a further particularly illustrativesummary of the most important film properties of the invention:

TABLE 1 Inventive particularly range preferred preferred Unit Testmethod Formulation Concentration of cycloolefin 2-60 4-50  6-40  %copolymer (COC) in base layer Glass transition temperature 70-270 90-250110-220 ° C. DIN 73 765 of cycloolefin copolymer (COC) Outer layer A COF<0.5 <0.45 <0.40 DIN 53 375 Average roughness R_(a) 200-1000 220-950 250-900 nm DIN 4768, cut- off of 0.25 mm Gloss, 60° <80 <70 <60 DIN 67530 Other film properties Whiteness >70 >75 >80 % BergerOpacity >55 >60 >65 DIN 53 146

The following measured values were utilized to characterize the rawmaterials and the films:

SV (Standard Viscosity)

Standard viscosity SV (DCA) is measured by a method based on DIN 53726in dichloroacetic acid.

Intrinsic viscosity (IV) is calculated as follows from standardviscosityIV(DCA)=6.67·10⁻⁴ SV(DCA)+0.118Coefficient of Friction (COF)

Coefficient of friction was determined to DIN 53 375. The coefficient ofsliding friction was measured 14 days after production.

Surface Tension

Surface tension was determined by what is known as the ink method (DIN53 364).

Roughness

Roughness R_(a) of the film was determined to DIN 4768 with a cut-off of0.25 mm.

Whiteness and Opacity

Whiteness and opacity were determined with the aid of the “ELREPHO”electrical reflectance photometer from the company Zeiss, Oberkochem(DE), standard illuminant C, 2° standard observer. Opacity is determinedto DIN 53 146. Whiteness is defined as W=RY+3RZ−3RX.

W=whiteness, and RY, RZ and RX=relevant reflection factors when the Y, Zor X color-measurement filter is used. The white standard used was abarium sulfate pressing (DIN 5033, Part 9). An example of a detaileddescription is given in Hansl Loos “Farbmessung” [Color Measurement],Verlag Beruf und Schule, Itzehoe (1989).

Light Transmittance

Light transmittance is measured using a method based on ASTM D1033-77.

Gloss

Gloss was determined to DIN 67 530. The reflectance was measured as anoptical value characteristic of a film surface. Based on the standardsASTM-D 523-78 and ISO 2813, the angle of incidence was set at 600. Abeam of light hits the flat test surface at the set angle of incidenceand is reflected and/or scattered by this surface. A proportionalelectrical variable is displayed representing light beams hitting thephotoelectronic detector. The value measured is dimensionless and mustbe stated together with the angle of incidence.

Glass Transition Temperature

Glass transition temperature T_(g) was determined using film specimenswith the aid of DSC (differential scanning calorimetry) (DIN 73 765) ADuPont DSC 1090 was used. The heating rate was 20 K/min and the specimenweight was about 12 mg. The glass transition T_(g) was determined in thefirst heating procedure. Many of the specimens showed an enthalpyrelaxation (a peak) at the beginning of the step-like glass transition.The temperature taken as T_(g) was that at which the step-like change inheat capacity—without reference to the peak-shaped enthalpyrelaxation—achieved half of its height in the first heating procedure.In all cases, there was only a single glass transition observed in thethermogram in the first heating procedure.

EXAMPLE 1 (INVENTIVE)

Chips of polyethylene terephthalate (prepared by the transesterificationprocess using Mn as trans-esterification catalyst, Mn concentration: 100ppm) were dried at 150° C. to a residual moisture below 100 ppm and fedto the extruder for the base layer B. Alongside this,chips of ® Topas6015 cycloolefin copolymer (COC) from Ticona (COC composed of2-norbornene and ethylene, see also W. Hatke: Folien aus COC [COCFilms], Kunststoffe 87 (1997) 1, pp. 58-62) with a glass transitiontemperature T_(g) of about 160° C. were also fed to the extruder for thebase layer B. The proportional amount of the cycloolefin copolymer (COC)in the base layer was 10% by weight.

Coextrusion followed by stepwise longitudinal and transverse orientationwas used to produce a white, opaque, three-layer film with ABCstructure, with a total thickness of 23 μm, and with one matt side. Thethickness of each of the outer layers is given in Table 2.

Matt outer layer A, a mixture of: 55.0% by weight of polyethyleneterephthalate with SV 800 = component (I) 15.0% by weight of component(II)* 30.0% by weight of masterbatch made from 95% by weight ofpolyethylene terephthalate (SV 800) and 5.0% by weight of ®Sylobloc 44 H(synthetic SiO₂ from Grace) Base layer B, a mixture of: 90.0% by weightof polyethylene terephthalate homopolymer with SV 800 10.0% by weight ofcycloolefin copolymer (COC) from Ticona, Topas 6015 Outer layer C, amixture of: 97.0% by weight of polyethylene terephthalate homopolymerwith SV 800  3.0% by weight of masterbatch made from 97.75% by weight ofpolyester (SV 800), 1.0% by weight of ®Sylobloc 44 H (synthetic SiO₂from Grace), and 1.25% by weight of ®Aerosil TT 600 (fumed SiO₂ fromDegussa). *Component (II) was prepared as described in more detail inExample 1 of EP-A 0 144 878.

The production conditions for each of the steps of the process were:

Extrusion: Temperatures base layer:     280° C. Temperature of take-offroll:      30° C. Longitudinal Temperature: 80 to 125° C. stretching:Longitudinal stretching ratio:      4.2 Transverse Temperature: 80 to135° C. stretching: Transverse stretching ratio:      4.0 Setting:Temperature:     230° C. Duration:      3 s

The film had the required good properties and the desired handling, andthe desired processing performance. The properties achieved in filmsproduced in this way are shown in Table 2.

EXAMPLE 2 (INVENTIVE)

Taking Example 1 as a basis, 50% by weight of regrind was now added tothe base layer. The amount of COC in the resultant base layer was again10% by weight. The process parameters were unchanged from Example 1.Visual observation was made of the extent of yellow discoloration of thefilm. It is seen from Table 2 that hardly any yellow discoloration ofthe film was visible.

EXAMPLE 3 (INVENTIVE)

Taking Example 1 as a basis, a film of thickness 96 μm was now produced.The amount of COC in the base layer was 8% by weight. The processparameters were unchanged from Example 1. Visual observation was made ofthe extent of yellow discoloration of the film. It is seen from Table 2that hardly any yellow discoloration of the film was visible.

Base layer B, a mixture of: 92.0% by weight of polyethyleneterephthalate homopolymer with SV 800  8.0% by weight of cycloolefincopolymer (COC) from Ticona, Topas 6015

EXAMPLE 4 (INVENTIVE)

Taking Example 3 as a basis, 50% by weight of regrind was now added tothe base layer B. The amount of COC in the base layer was again 8% byweight. The process parameters were unchanged from Example 1. Visualobservation was made of the extent of yellow discoloration of the film.It is seen from Table 2 that hardly any yellow discoloration of the filmwas visible.

COMPARATIVE EXAMPLE 1

Example 1 of DE-A 23 53 347 was repeated. The example was modified byadditionally including 50% by weight of regrind in the process. It isseen from Table 2 that there was marked visible yellow discoloration ofthe film.

Base layer B, a mixture of: 47.5% by weight of polyethyleneterephthalate homopolymer with SV 800 50.0% by weight of regrind of thesame material (95% by weight of polyester + 5% by weight ofpolypropylene)  2.5% by weight of polypropylene

COMPARATIVE EXAMPLE 2

Example 1 of EP-A 0 300 060 was repeated. The example was modified byadditionally including 50% by weight of regrind in the process. It isseen from Table 2 that there was marked visible yellow discoloration ofthe film.

Base layer B, a mixture of: 45.0% by weight of polyethyleneterephthalate homopolymer with SV 800 50.0% by weight of regrind of thesame material (90% by weight of polyester + 10% by weight ofpolypropylene)  5.0% by weight of polypropylene

COMPARATIVE EXAMPLE 3

Example 1 of EP-A 0 360 201 was repeated. The example was modified byadditionally including 50% by weight of regrind in the process. It isseen from Table 2 that there was marked visible yellow discoloration ofthe film.

Base layer B, a mixture of: 40.0% by weight of polyethyleneterephthalate homopolymer with SV 800 50.0% by weight of regrind of thesame material (80% by weight of polyester + 20% by weight ofpolypropylene) 10.0% by weight of polypropylene

COMPARATIVE EXAMPLE 4

Example 1 of DE-A 195 40 277 was repeated. The example was modified byadditionally including 50% by weight of regrind in the process. It isseen from Table 2 that there was marked visible yellow discoloration ofthe film.

Base layer B, a mixture of: 43.5% by weight of polyethyleneterephthalate homopolymer with SV 800 50.0% by weight of regrind of thesame material (87% by weight of polyester + 13% by weight ofpolystyrene)  6.5% by weight of polystyrene

TABLE 2 Glass Coefficient Film Layer Additive transition Gloss offriction COF Average thick- structure Polymer concentration temperatureWhite- Assessment Outer Side A with roughness R_(a) Ex- ness and layeradded to in base layer of additive ness Opacity of film layer respect toSide A Side C ample μm thicknesses polyester % by wt. ° C. % %yellowness A side C nm nm E1 23 ABC COC 10 170 75 75 ++ 45 0.3 350 120(2/19/2) E2 23 ABC COC 10 170 76 80 + 45 0.3 350 110 (2/19/2) E3 96 ABCCOC 8 170 85 85 ++ 40 0.3 330 100 (2/92/2) E4 96 ABC COC 8 170 86 90 +40 0.3 320 98 (2/92/2) CE1 155 B Poly- 5 −10 80 70 − 46 0.45 410 410propylene CE2 100 B Poly- 10 −10 88 80 − 57 0.45 180 180 propylene CE3100 ABA Poly- 20 −10 92 89 − 54 0.25 370 370 propylene CE4 125 B Poly-13 100 82 82 − 51 0.35 480 480 styrene Key to yellowness of filmsproduced: ++: no yellow coloration discernible +: slight yellowcoloration discernible −: marked yellow coloration discernible

1. A white, biaxially oriented and coextruded polyester film with atleast one matt side and embracing at least one base layer B and at leastone matt outer layer A, both made from polyester, wherein at least thebase layer comprises, based on the weight of this layer, from 2 to 60%by weight of cycloolefin copolymer (COC), where the glass transitiontemperature T_(g) of the COC is in the range from 70 to 270° C., whereinthe matt outer layer A exhibits a roughness, expressed as R_(g), in therange from about 200 to 1000 nm and the value measured for gas flow inthe range from about 0 to about 50 g.
 2. The white polyester film, asclaimed in claim 1, wherein the COC comprises polynorbornene,poly-dimethylocahydronaphthalene, polycyclopentene orpoly-5-methylnorbornene.
 3. The white polyester film, as claimed inclaim 1, wherein the COC has a glass transition temperature T_(g) in therange from about 90 to about 250° C.
 4. The white polyester film asclaimed in claim in 1, wherein a matt outer layer A has been arranged onthe COC-containing base layer B, and comprises additive in the form ofinert inorganic antiblocking agents and, comprises a blend or a mixturemade from two components (I) and (II), and wherein component (I) of themixture or of the blend is an ethylene terephthalate homopolymer orethylene terephthalate copolymer or a mixture made from these, andwherein component (II) of the mixture or of the blend is an ethyleneterephthalate copolymer which is composed of the condensation product ofthe following monomers or of their derivatives capable of formingpolyesters: A) from about 65 to about 95 mol % of isophthalic acid; B)from about 0 to about 30 mol % of at least one aliphatic dicarboxylicacid having the formula HOOC(CH₂)_(n)COOH, where n is in the range from1 to 11; C) from about 5 to about 15 mol % of at least one sulfomonomercontaining an alkali metal sulfonate group on the aromatic moiety of adicarboxylic acid; D) a copolymerizable aliphatic or cycloaliphaticglycol having from 2 to 11 carbon atoms, in the stoichiometric amountnecessary to form 100 mol % of condensate; where each of the percentageis based on the total amount of monomers forming component (II), andwherein the weight ratio of the two components (I) and (II) of themixture for the outer layer A, or of the blend, is in the range fromabout (I):(II)=10:90 to about (I):(II) 95:5.
 5. The white polyesterfilm, as claimed in claim 1, wherein the total thickness of the film isin the range from about 4 to about 400 μm, and wherein the thickness ofthe outer layer(s) is in the range from about 0.1 to about 10 μm, whereouter layers applied on the two sides may be identical or different intheir thickness and makeup.
 6. The white polyester film, as claimed inclaim 1, wherein the whiteness of the film is above about 70%.
 7. Thewhite polyester film, as claimed in claims 1, wherein the opacity of thefilm is above about 55%.
 8. The white polyester film, as claimed inclaim 1, wherein the gloss of the film on the surface of the outer layerA is below about
 80. 9. The white polyester film, as claimed in claim 8,wherein an intermediate layer has been arranged between theCOC-containing base layer B and the matt outer layer A and has athickness in the range from about 0.5 to about 15 μm.
 10. The whitepolyester film, as claimed in claim 1, wherein the base layer B alsocomprises, in mach case based on the total weight of the base layer B,from about 0.5 to about 25% by weight of one or more compounds selectedfrom vacuole-inducing compounds other than COC, white fillers andpigments.
 11. A process for producing a white polyester film with atleast one matt side, as claimed in claim 1, in which the meltscorresponding to the individual layers of the film are coextrudedthrough a flat-film die, the resultant film is drawn off on one or morerollers for solidification, the film is then biaxially stretched(oriented), and the biaxially stretched film is heat-set and is thenwound up, which process comprises carrying out the biaxial stretching ina sequence, first stretching longitudinally and then transversely, andwhich comprises carrying out the longitudinal stretching at atemperature in the range from about 80 to about 13° C. and the trasversestretching at a temperature in the range from about 90 to about 150° C.,and which comprises setting the longitudinal stretching ratio in therange from about 2.5:1 to about 6:1, and the transverse stretching ratioin the range from about 3.0:1 to about 5.0:1.
 12. A process forproducing a white polyester film, as claimed claim 1, in which the meltscorresponding to the individual layers of the film are coextruded ugh aflat-film die, the resultant film is drawn off on one or more rollersfor solidification, the film is then biaxially stretched (oriented), andthe biaxially stretched film is heat-set and is th wound up, whichprocess comprises carrying out the stretching in a simultaneousstretching frame, and which comprises setting the stretching temperatureto about ≦125° C.
 13. The process as claimed in claim 11 or 12, whereinthe film is held for a period of from about 0.1 to about 10 s at atemperature in the range from about 150 to about 250° C. afterstretching for heat-setting, and wherein the film is then cooled andwound up.
 14. The process sa claimed in claim 11 or 12, wherein, afterheat-setting and prior to winding-up, the film is chemically treated orcorona-, or flame-treated where the intensity of treatment is adjustedso that the surface tension of the film after treatment is greater thanor equal to about 45 mN/m.
 15. The process as claimed in claim 11 or 12,wherein cut material directly associated with the production of the filmis introduced back into the production process as regrind in amounts inthe range from about 10 to about 70% by weight based on the total weightof the film.
 16. A method of making a packaging for foods or otherconsumable items which are sensitive to light or to air or light andair, a hot-stamping foil, a label film, an image-recording paper, aprinted sheet or a magnetic recording card, which method comprisesconverting a film according to claim 1 into a packaging for foods orother consumable items which are sensitive to light or to air or lightand air a hot-stamping foil, a label film, an image-recording paper, aprinted sheet or a magnetic recording card.